Define __all__ for torch.utils.tensorboard (pytorch#147550)

Fixes the issue:

```python
import torch.utils.tensorboard
torch.utils.tensorboard.FileWriter  # pyright: "FileWriter" is not exported from module "torch.utils.tensorboard"
torch.utils.tensorboard.RecordWriter  # pyright: "RecordWriter" is not exported from module "torch.utils.tensorboard"
torch.utils.tensorboard.SummaryWriter  # pyright: "SummaryWriter" is not exported from module "torch.utils.tensorboard"
```

The [docs page for `torch.utils.tensorboard`](https://pytorch.org/docs/stable/tensorboard.html)
Pull Request resolved: pytorch#147550
Approved by: https://github.com/albanD

Author: ringohoffman

torch/utils/tensorboard/__init__.py

@@ -9,5 +9,11 @@
 del Version
 del tensorboard
 
-from .writer import FileWriter, SummaryWriter  # noqa: F401
-from tensorboard.summary.writer.record_writer import RecordWriter  # noqa: F401
+from .writer import FileWriter, SummaryWriter
+from tensorboard.summary.writer.record_writer import RecordWriter
+
+__all__ = [
+    "FileWriter",
+    "RecordWriter",
+    "SummaryWriter",
+]